The cathode of a normal glow discharge tube is usually contained within a glass envelope and typically is constructed in the form of a cylinder open at both ends. The metal or other electrically conductive material of which a cathode is constructed has appropriate cathodic properties, namely a resistance to sputtering and a good electron emission characteristic. As is well known, the principal mechanism of operation of a cathode is the release of electrons by the impact thereon of high energy positive ions, the released electrons being repelled by the cathode on the electric field or discharge.
Unfortunately, the impact on the cathode of high energy positive ions can also cause the release of atoms of the metal of which the cathode is constructed, which atoms may be deposited on areas in the vicinity of the cathode. This process is known as sputtering and is well known to be deleterious to cathode operation. The action of sputtering exposes fresh metal on the cathode which might absorb certain gases present in the glow discharge device and hence give rise to a gettering effect, the absorbed gases possibly becoming buried if some of the sputtered material is redeposited on the cathode.
It has been found that sputtering is prevalent when the cathode current density is high and so a cathode having a configuration which gives rise to high current density regions or "hot spots" will tend to exhibit sputtering and gettering characteristics, the former being undesirable but the latter advantageous. High current density regions have been found to be associated with discontinuous surfaces on a cathode such as are presented by corners or points, for example. Thus it is desirable to avoid discontinuous surfaces in a cathode in order to maintain the electric field in the glow discharge device at as uniform a value as possible.
In the case of a ring laser gyroscope, it is usual to dispense with the glass envelope so that the cathode itself forms the envelope which needs to be gas tight and to have an opening for communication with the lasing path of the gyroscope. Furthermore, the cathode has to be sealed in a gas tight and physically secure manner to a block or other construction in which the lasing path is provided. Thus the cylinder open at both ends used as a cathode in other glow discharge devices as discussed above is of no use in a ring laser gyroscope. If one end of the cylinder were to be closed by an end cap or plate extending generally transverse to the longitudinal axis, this would give rise to a discontinuous surface within the cylinder due to the "corner" at the transition between the end cap or plate and the cylindrical body. It will be appreciated that such a discontinuity can be minimised by curving the interface between the cylindrical body and the end cap and indeed cathodes are known having cylindrical bodies with hemispherical ends.
However, there is another important consideration appertaining to cathodes, namely discharge "noise" which is created by oscillations in the glow discharge. In a laser gyroscope, discharge noise imposes itself on the laser beams and, depending on the severity, may firstly manifest itself as a significant content of the gyroscope output signal giving rise to a degraded gyroscope performance, and as a factor making the control of the round trip path length of the laser difficult to effect. It is important that the round trip path length of the laser in a laser gyroscope is controlled within very stringent limits because change in path length is the essence of operation of such a gyroscope, whereby changes due to any factor other than that which is to be measured (i.e. that due to rotation of the gyroscope about its sensitive axis) have to be eliminated. In the context of discharge noise, it has been found that cathodes with cylindrical bodies and curved or hemispherical ends exhibit an inferior characteristic in this respect albeit they have a desirable low sputtering characteristic as discussed above. In contrast, conical and square-ended cathodes have very good discharge noise characteristics but high sputtering characteristics. Thus, to date, there has existed a direct conflict between the sputtering and discharge noise characteristics of a cathode for a given configuration thereof.